Spherical elastomeric bearings permit motion around three mutually perpendicular axes through elastomer shear loading, while high loads are reacted axially in elastomer compression. Some radial loading can be accommodated, depending on the degree of "wrap-around" (spherical radii versus outside diameter). Conical elastomeric bearings, have the ability to react high loads in both the axial and radial directions, through compression and shear of the elastomer, while accommodating torsional motions in elastomer shear. The axial and radial stiffnesses are normally quite high, depending on the included angle of the cone, while the torsional mode has a relatively low stiffness. (Lastoflex.RTM. Bearing Design Guide, Report No. PE 76-006, pages I-11 and I-12, issued Jan. 30, 1976 by the Product Engineering Department of Lord Kinematics, Erie, Pa.)
Much thought has been devoted to the subject of improving fatigue life in elastomeric bearings. A basic problem is that the loads are not evenly distributed from laminate-to-laminate. For the elastomer laminates, this means that a particular laminate will fail first. (The normal mode of deterioration is a slow abrasion of the elastomer which can be visually monitored.)
U.S. Pat. No. 4,435,097 (Peterson, 1984) adequately describes the problem. "A significant commercial variety of bearings is characterized by the alternating bonded lamellae being disposed concentrically about a common center, i.e., so that successive alternating layers of resilient and nonextensible materials are disposed at successively greater radial distances from the common center. This variety of bearings includes a number of different configurations, notably bearings which are cylindrical, conical or generally spherical in shape or which are essentially sectors of cylinders, cones and spheres. Such bearings typically are used in aircraft, especially as rotor shaft supports in helicopters. As noted in U.S. Pat. No. 3,679,197 (Schmidt, 1972), bearings of this type frequently are required to accommodate cyclic torsional motion about a given axis while simultaneously carrying a large compressive load along that axis, with the result that greater compressive stresses and shear stresses and strains are established in the resilient layers closest to the common center and failure from fatigue encountered in accommodating the torsional motion tends to occur at the innermost resilient layer. Schmidt proposed to improve the fatigue life of such bearings by progressively increasing the thicknesses of successive layers of resilient material with increasing radius and simultaneously to progressively decrease the modulus of elasticity of those same layers with increasing radius. However, the Schmidt technique is expensive in that it requires that each elastomer layer be made of a different material. Thus an elastomeric bearing consisting of fifteen resilient layers necessitates provision of fifteen different "elastomer materials." Peterson discloses forming at least some of the elastomer laminates of at least two different elastomer stocks having different elasticity characteristics.
The above-described techniques for improving bearing fatigue life each tackle the problem from the viewpoint of matching the elastomer, on a laminate-by-laminate basis, to the stress distribution throughout the bearing. A more subtle problem exists in that stress distribution is not even throughout a given laminate, especially in an asymmetrically loaded bearing.